There's a fat wire clip that fits in one of three grooves on the shock body under the spring collar. You have to compress the spring to expose the clip, remove the clip then slowly release the spring.
You'll want to make a rig similar to Gizmows in order to minimize the risk of it slipping. It's a fairly safe practice with the right stuff but if you mickeymouse it and the spring flies off while compressed it could do ya some dirt.
I'm assuming you want to paint the spring. If you use automotive style spring compressors you'll probably rip the paint right back off of the spring when you reinstall it.
Your local dirtbike shop may be able to pop it off in a few seconds if they do shock work. Might get the whole thing over pretty quick so you can just move on.
Most Tw's are set on the middle clip position frm the factory. When you put it back together you should consider moving the clip to the highest setting when you reassemble the shock. That swingarm is awfully long and will place more leverage on the shock. There's a slight increase in preload available by moving the clip.
Just as reference, my 2" stretch softened up the rear shock considerably. If it were any longer or if I were any heavier I'd be dealing with pretty serious bottoming offroad. In my case it tamed the shock perfectly.
Just how much of a factor that will be on a street only bike I don't know. But I'm curious. Maybe the other hollow mod builders with longer swingarms will ring in.